Battery separator and method for manufacture thereof



FIG.

Flam GLASS 70 p 10, 1963 w. G. SMITH ETAL 3,103,461

BATTERY SEPARATOR AND METHOD FOR MANUFACTURE THEREOF Filed Dec PIC-.5

INVENTORS mam/r1 a: sM/rflu mew/Me .1 many 3,103,461 Patented Sept. 10, 1963 3,103,461 BATTERY SEPARATOR AND METHOD FOR MANUFACTURE THEREOF William G. Smith, Sarver, and Arthur J. Marcozzi, Blairsville, 1%., assignors to Pittsburgh Plate Glass Compauy, Allegheny County, Pa.

Filed Dec. 19, 1958, Ser. No. 781,718 6 Claims. (Cl. 162-116) This invention relates to battery separators and it has particular relation to battery separators made of glass fibers.

It is known that glass wool mats can be used as retainer mats in a lead-acid storage battery. Glass wool in its commercial forms is too porous when formed in a thin mat to permit its use by itself as a battery separator. It does not prevent lead sulfate crystal growth between the electrodes in the battery, which is commonly known as treeing. The glass wool mats must be used in cornbination with other materials such as microporous rubber, wood or phenolic resin-impregnated paper, which other materials act as the separator to prevent treeing in the battery.

It is an object of this invention to provide a glass fiber battery separator which consists essentially of glass fibers and a binder therefor in such form that the fibers provide a microporous separator which permits passage of the electrolyte in the battery through it and vertical flow of gases formed during the battery reactions, but does not permit treeing. It is desired that the glass fiber battery separator have a low electrical resistance and that it permits proper operation of the battery under all normal uses of a battery.

It is a further object of the invention that the battery separator be sturdy, yet flexible, so as to permit handling and assemblage of the separator in the battery. A glass fiber separator having these properties has the advantage that it is more resistant to deterioration by battery acids than cellulose-containing separators and therefore provides longer life for the battery and is less expensive than microporous rubber separators.

In accordance with the present invention, a glass fiber battery separator is prepared by milling glass fibers to reduce their length so that about 90 percent or more by weight of the fibers have an average length of 0.008 to 0.125 inch and depositing an aqueous dispersion of the milled fibers on the surface of a foraminous screen. The deposition of the fibers on the screen results in the formation of a layer of fibers in felted, haphazard, overlapping relation to each other on the screen. The water is pulled through the openings in the screen during the formation of the layer with sufiicient force to cause a portion of the short fibers to project through the openings in the screen at an angle to the plane of the layer of felted fibers. This forms a series of spaced projections on one surface of the felted layer and a series of continuous channels running across the surface of the felted layer between the projections.

The fibers are bonded together in this manner by a resinous binder. The binder may be applied in aqueous solution or dispersion with the fibers or as a separate application after the fibers have been deposited on the screen. A separate application of the binder permits greater control of the dispersion and deposition of the fibers and greater control of the application of the binder. The binder is present in an amount which is sufficient to bind the fibers together but not form a continuous film between the fibers and thereby prevent the formation of a microporous layer. There should be a sufficient amount of binder in the fibrous separator to provide strength and flexibility to the separator for handling in the formation of the battery and during use of the battery. The amount of binder may vary between 10 to 30 percent by weight of the fibers in the separator.

The glass fiber separator and its method of manufacture can be further described by reference to the drawing in which:

FIG. 1 is a diagrammatic view of an apparatus suitable for making the separator;

Fl G. 2 is an elevation of a coater adapted for depositing an aqueous dispersion of fibers on a screen;

FIG. 3 is a side view of FIG. 2;

FIG. 4 is an isomeric view of the battery separator;

FIG. 5 is a view in section taken along lines VV of FIG. 4;

FIG. 6 is an isometric view of another form of the battery separator, and

FIG. 7 is a view in section taken along lines VIIVII of FIG. 6.

In the practice of the invention, staple glass fibers such as may be formed by the introduction of a glass rod into a high temperature, high velocity blast as described in US. Patent No. 2,489,243 may be used to form the short, milled fibers. These fibers may have an average diameter of 1 to 5 microns and an average length of about V2 to 10 or more inches. Other types of glass fibers may be used as a starting material and may vary in composition. The composition of a typical glass fiber which is useful as a starting material is as follows.

Ingredient: Percent by weight SiO 59.2-59.8 Na o 14.6-14.95 A1 0 n 3.754.8 ZrO 4.04.2 B 0 7.5-7.9 TiO 8.258.9 Fe o 0.04-0.14

The staple glass fibers 10 and water are introduced into a Bauer disc mill 11 which grinds the fibers up so that they are reduced in length so that about percent or more by weight of the fibers have an average length of 0.008 to 0.125 inch. The ratio of the length of the milled fibers 12 to the diameter is such that 90 percent by weight or more of the fibers have an average length to diameter ratio range of :1 to 500:1. The disc mill is composed of a housing 13 in which are mounted two alundum discs 14 and 15 which are in slightly spaced, face-to-face relation to each other. One of the discs 14 is stationary While the other disc 15 is mounted on a shaft 17 connected to a motor 19. The degree of grinding and reduction in length of the fibers is controlled by the distance between the faces of the discs, the power supplied to the rotating disc 15 and the length of time of grinding. By way of example, the discs, which may be 8 inches in diameter, may be about 9.001 inch apart and disc 15 may be rotated at about 3400 rpm.

The short, milled fibers 12 are deposited in container 20 in which they are dispersed and maintained in this condition by means of a conventional pulping stirrer 25 such as a Waring Blendor. The aqueous carrier which is used to flush the milled fibers 12 from the mill 11 is adjusted to a pH of 3 to 5 with a suitable amount of citric acid. Other inorganic acids such as sulfuric or bydrochloric acids can be used, but they may cause corrosion of some of the equipment. The water in the container 20 is also maintained at a pH of 3 to 5. The beating of the fibers in the acidified aqueous medium by means of the stirrer 25 gives a more uniform dispersion of the fibers. The fibers seem to repel each other so that each fiber is separately dispersed in the Water and bunches of fibers are not likely to occur. It has also been fiound that the adhesion of the fibers to each other after deposition on the screen is increased by the wetting of the fibers with the acidified water solution. The amount of fibers in the aqueous acid dispersion is about 0.25 to 2 percent by weight of the dispersion with percent based on the Weight of the dispersion being a preferred concentration of fibers. Sufficient acid should be added to produce the desired pH in the aqueous dispersion of the fibers. The amount of acid employed will depend on the type of acid, the composition of the glass and the initial pH of the water.

The aqueous dispersion of short, milled fibers is pumped through a cylindrical line 28 by means of a pump 30 under a pressure of to 25 pounds per square inch at a volume of to 30 gallons per minute into a coater 32. The coater is in the form of three annularly disposed cylinders 34, 36 and 38 with the cylinder 34 being an extension of the line 28. The cylinder 36 surrounds the cylinder 34 and is closed at its ends by end walls 40. These end walls have circular, centrally disposed openings therein to permit the passage of cylinder 34 through the cylinder 36 and form an enclosed annular space between cylinders 34 and 36. Cylinder 38 encloses cylinder 36 and its end walls 42 join with cylinder 36 to form an enclosed annular space between cylinders 36 and 38. The cylinder 34 has a series of openings 44 in it along its bottom side through which a portion of the aqueous dispersion from line 28 can pass.

The dispersion flows downwardly through these openingjs 44 and then upwardly through the annular space formed by the walls of the cylinders 34 and 36 to a slit 46 extending along the length of the top surface of cylinder 36. The aqueous dispersion passes through this slit and then downwardly through the annular passage provided by the Walls of the cylinders 36 and 38 and out through a slit 48 extending along the length of the bottom side of the cylinder 38. The dispersion then flows into another cylinder 50 which is attached to the bottom side of cylinder 38. The aqueous dispersion flows from the cylinder 50 through a slit 52 which extends along the bottom surface of cylinder 50. One side wall 53 extends slightly lower than the other side wall 54 of the cylinder 50. The bottom of side wall 53 almost touches a conveyor 60, whereas the bottom of side wall 54 is slightly higher by about the thickness of the layer of fibers to be deposited.

The cylinder 34 extends beyond the length of its enclosing cylinder 36 and forms into a line 61 which carries and recirculates the dispersion to line 28 at a point near the attachment of the line 28 with the cylinder 34. The recirculating line 61 provides a control to insure a steady flow of the dispersion through the openings 44 in cylinder 34 and a continual steady flow through the coater. It is important that a steady flow be maintained in order to deposit a uniform thickness of fibers on the foraminous conveyor screen 60 which passes under the coater 32 at a speed of about 20 to feet per minute. The speed of the conveyor may be varied depending upon the thickness of layer of fibers which is desired.

Directly below the coater 32 and on the other side of the conveyor screen 60 is suction tank 62. The opening 64 in the tank 62 is directly beneath and in line with the opening 52 of the coater 32 and is about /8 inch Wide. The excess water of the dispersion is pulled through the foraminous conveyor screen 60 and the short fibers are caused to deposit in closely felted relationship to form layer 66 on the screen. The vacuum which is applied by means of the tank 62 may vary from A to 1% inches of mercury, and it is sufiicient to cause a portion of the milled fibers 12 to extend through the openings in the screen at an angle of about 15 to 90 to the general plane of the layer 66 of the fibers on the screen 60 and form a series of projections 68. The suction is adjusted so that the fibers are not pulled through the openings with such force that there remains starved areas or areas of thickness in the layer 66 of felted fibers which are substantially less than the average thickness of the layer 66 without the projections 68. The projections 68 extend. from the layer 66 for a distance equal to about onehalf to the same as the thickness dimension of the layer itself. Usually the top surface of layer 66 of felted fibers is fairly smooth; however, some uneveness is permissible.

The water which has been pulled through the conveyor screen is returned by line 69 to a tank 70 where the pH is again adjusted by means of citric or other acid to 3 to 5. A portion of this water may be used to flush the milled fibers through the disc mill 11 and a portion to replenish the water in container 20.

The layer 66 of short fibers on the screen 60 is then caused to pass under a binder coater 72 where an aqueous dispersion of a resinous binder is deposited on the layer 66 of fibers. The aqueous solution of binder passes through the layer 66 and projections 68 and through the conveyor screen 60 to tank 74 located directly underneath the coater 72 and on the other side of the screen 69 from the coater 72. The water and binder are caused to flow through the layer 66 and projections 68 and into the tank 74 by means of a vacuum in the tank 74 of A to 1V4 inches of mercury. A portion of the binder is retained in the fibers in the layer 66 and projections 68. The water and binder are returned from tank 74 by way of line 76 to a binder mixing tank 78. Here. resin is added to the aqueous dispersion for use again in the coater 72. The pH of the binder solution may be also controlled to provide the best dispersion of binder if a particulated binder such as a latex is employed. For example, synthetic latices, such as polystyrene and buta-dienc-styrene latices, are best maintained in dispersed condition at a pH of 9 to 11. Control of the pH may not be necessary if a water soluble binder is employed.

It is preferred that the binder be in particle form such as the form of synthetic latices which are prepared by aqueous emulsion polymerization of ethylenic monomers, although other forms of synthetic resinous binders which are inert with respect to lead-H 50 battery reactions are suitable. Water soluble, inert organic resins such as the water soluble phenolic resins are contemplated as well as the latices which are prepared by aqueous emulsion polymerization of ethylenic monomers such as the various acrylates which are esters of acrylic and methacrylic acid and an aliphatic alcohol having 1 to 6 carbon atoms including, for example, methyl methacrylate and methacrylate, vinyl chloride, styrene, acrylonitrile, chloro-vinyl acetate, buta diene, vinylidene chloride and various copolymers of the above monomers such as butadiene-acrylonitrile, vinyl chloride-vinyl acetate, butadiene-styrene copolymers and like materials which can be employed in latex form as glass fiber binders. The latices generally have an average par ticle size of 0.1 to 5 microns.

A plasticizcr is employed with latices which tend to deposit as brittle or discontinuous films. For example, a plasticizer is generally used with latices of polyvinyl acetate, polyvinyl chloride, a polyacrylate and polystyrene; whereas, a plasticizer is generally not used with a butadiene-styrene latex. The plasticizer may be any known plasticizer for the various resins such as dibutyl phthalate, tricresyl phosphate, dioctyl phthalate, diisooctyl phthalate and other esters which are conventionally used as plasticizers.

A combination of resins which has been found to be useful to provide the desired properties to the separator is that of polystyrene and butadiene-styrene latices. The polystyrene is the principal binder and the butadiene-styrene is present in amounts of 5 to 10 percent by weight of the polystyrene to provide added flexibility and strength to the separator. A typical binder solution is as follows.

3,103,461 6 V V Ingredient: Parts by weight Although the present invention has been described with Aqueous polystyrene latex containing 40% by respect to specific details of certain embodiments thereof, weight of solids 38 it is not intended that such details be limitations upon the Aqueous butadiene-styrene latex containing 48 by weight of solids with the molar ratio of the butadiene to styrene being 40 to 60 2.8 Plasticizer (butyl benzyl phthalate) 4.9 Water 54.3

Ordinarily, the aqueous binder solution may contain to percent by weight of resinous solids. The viscosity of the solution should be within about 4 to 8 centipoiscs at 20 C. and the pH should be 9 to 11.

The layer of fibers 66 with the binder distributed therein is next carried by conveyor screen 60 to an oven 80 in which the binder is heated to cure it and bond the fibers together. Just prior to and/ or after layer 66 passes under the coater 72, its upper surface may be calendered by a hot roll (not shown) to produce a smooth upper surface to the layer. The layer 66 emerges from the oven 80 with the fibers which have projected through the openings in the screen bonded to form a permanent pattern of projections 68 on the lower surface of the layer. The layer 66 as thus formed is cut into strips by a circular, rotating slitter 82 and thereafter chopped into small rectangular pieces by a chopper 84.

Battery separators 90 and 91 which can be produced in this manner are shown in FIGS. 4, 5, 6 and 7. As shown in FIGS. 4 and 5 the separator 90 is formed of the short fibers 12 in closely felted, haphazard relationship with a uniform pattern of pyramid-like or dome-like projections 68 extending from one surface of the layer of felted fibers. These spaced, peaked, pyramid shaped projections 68 form channels 94 which run between the projections 68 continuously along the one surface of the separator from one edge to the opposing edge. The projections 68 are formed by the short, milled fibers which are inclined at an angle of about 15 to 90 to the general plane of the separator 90 and are bonded in this relationship by the resinous binder. The height of the projections will be about the same or slightly less than the thickness of the continuous layer portion 66 of the separator. The density of the fibers in the projections will be substantially the same as in the continuous portion of the separator but may be slightly greater so as to insure stability of these portions. The density of the fibers in the separator may be between 8 to 15 pounds per cubic foot. The density in the projections may be greater than the density of the continuous phase of the separator by having a greater binder content therein. This may be achieved by applying the heat in the oven from the under side of the conveyor, i.e. the side closest to the projections, during the curing of the binder. This appears to cause the binder in the fibers to migrate into the projections.

The size and shape of the projections may be varied by the form of the conveyor screen 60. A screen having a mesh of 14 has been found to produce satisfactory projections 68. This results in projections 68 which have a peak-to-peak distance of 0.071 inch, which is a distance greater than the projection height. Instead of being in the form of pyramids as shown in FIG. 5, the projections may be in the form of cones, domes, truncated pyramids or cones, or elongated peaked ridges such as the ridges 96 shown in separator 91 in FIGS. 6 and 7. These ridges may be produced by having the fill or transverse wires of the screen spaced a greater distance apart than the warp or longitudinal wires of the screen or vice versa. It is obvious that other patterns may be formed which patterns will provide the continuous channels 94 across the face of the battery separator to permit the passage of gases which are formed during the battery reactions. The overall thickness of the separators is between 0.068 to 0.086 inch and in most cases 0.07 to 0.08 inch. Of this thickness the projections may range in height from 0.02 to 0.04 inch and the continuous layer may range from 0.035 to 0.05 inch.

scope of the invention except insofar as included in the accompanying claims.

We claim:

1. A method of making a microporous glass fiber mat which comprises depositing on a foraminous screen an aqueous dispersion of short glass fibers, percent or more by Weight of which have an average length of 0.008 to 0.125 inch to form a continuous layer having a thickness from 0.035 to 0.050 inch formed of fibers in felted, haphazard, overlapping relation, pulling the Water of the dispersion through the openings during deposition in the screen with suificient force to cause a portion of the fibers to extend through the openings in the screen at an angle to the plane of the layer of felted fibers and to form a series of peaked projections extending from the layer a distance less than the distance between the members in the foraminous screen and a distance from about onehalf to the same as the thickness of the layer, the projections defining continuous channels on one surface of the felted layer and binding the fibers in the layer and in the projections together.

2. A method of making a microporous glass fiber mat which comprises preparing an aqueous dispersion of short glass fibers, 90 percent or more by weight of which have an average length of 0.008 to 0.125 inch, the pH of the dispersion being 3 to 5, depositing the aqueous dispersion of fibers on a foraminous screen to form a continuous layer having a thickness from 0.035 to 0.050 inch formed of the fibers in felted, haphazard, overlapping relation, pulling the water of the dispersion through the opening during deposition in the screen with sufficient force to cause a portion of the fibers to extend through the openings in the screen at an angle to the plane of the layer of felted fibers and form a series of peaked projections extending from the layer a distance less than the distance between the members in the foraminous screen and a distance from about one-half to the same as the thickness of the layer, the projections defining continuous channels on one surface of the felted layer, preparing an aqueous dispersion of a particulated resinous binder, the dispersion having a pH of about 9 to ll, applying the dispersion of binder to the layer of fibers, pulling the water and the binder through the layer of fibers to distribute the binder in the layer and projections and binding the fibers together in this form.

3. A method of making a microporous glass fiber mat which comprises preparing an aqueous dispersion of short glass fibers, 90 percent or more by weight of which have an average length of 0.008 to 0.125 inch, the dispersion having a pH of 3 to 5, depositing the dispersion on a foraminous screen to form a continuous layer having a thickness from 0.035 to 0.050 inch formed of fibers in felted, haphazard, overlapping relation, pulling the water of the dispersion through the openings during deposition in the screen with sufiicient force to cause a portion of the fibers to project through the openings in the screen at an angle to the plane of the layer of felted fibers and form a series of peaked projections extending from the layer a distance less than the distance between the members in the forarninous screen and a distance from about one-half to the same as the thickness of the layer, the projections defining continuous channels on one surface of the felted layer, depositing a binder in an aqueous medium on the layer of felted fibers, pulling the water through the layer of fibers to distribute the binder in the layer and projections therefrom and heating the fibers and binder distributed therein by means of a heat source located on the side of the layer having the projections extending therefrom to dry and bind the fibers in the layer and projections in this form.

4. A waterlaid microporous glass fiber mat suitable for use as a battery separator consisting essentially of a continuous layer having a thickness of from .035 to .050 inch of short glass fibers, including a high proportion of fibers of lengths ranging from 0.008 to 0.125 inch, arranged in overlapping, haphazard, felted relation to each other with one exposed surface of the layer having a plurality of small, spaced, peaked projections made up of the fibers extending from the layer at an angle to the plane of the layer, the projections being arranged to provide a plurality of continuous channels extending across the surface of the mat between the projections, the projections extending from the layer a distance from about one-half to the same as the thickness of the layer and having their peaks spaced apart a distance greater than the projection height, the fibers in the layer and projections being bonded together by a binder.

5. A waterlaid microporous glass fiber rnat suitable for use as a battery separator consisting essentially of a continuous layer having a thickness of from .035 to .050 inch of short milled glass fibers 90 percent or more by weight of which have an average length of 0.008 to 0.125 inch, arranged in overlapping haphazard, felted relation to each other with one exposed surface of the layer having a plurality of small, peaked projections, the peaks of the projections being spaced apart a distance greater than the projection height, the projections being made up of fibers extending from the layer at an angle to the plane of the layer, the projections extending from the layer a distance of from about one-half to the same as the thickness of the layer and being arranged to provide a plurality of continuous channels extending across the surface of the mat between the projections, the fibers in the layer and projections being bonded together by a binder.

6. A waterrlaid microporous glass fiber mat suitable for use as a battery separator consisting essentially of a continuous layer having a thickness from 0.035 to 0.050 inch and made of short milled fibers, 90 percent or more by weight of which have an average length of from 0.008 to 0.125 inch arranged in overlapping, haphazard, felted relation to each other with one exposed surface References Cited in the file of this patent UNITED STATES PATENTS 910,358 Chapman Jan. 10, 1909 1,595,637 Wheildon Aug. 10, 1926 1,712,852 Streeter May 14, 1929 1,732,540 Say ler Oct. 22, 1929 2,117,371 Slayter May 17, 1938 2,428,370 Powers Oct. 7, 1947 2,677,007 Jensen Apr. 27, 1954 2,771,363 Fish Nov. 20, 1956 2,787,542 Lubino Apr. 2, 1957 2,901,390 Conklin Aug. 25, 1959 FOREIGN PATENTS 741,675 Great Britain Dec. 7, 1955 805,407 Great Britain Dec. 3, 1958 226,616 Australia Aug. 21, 1958 OTHER REFERENCES O'Leary et 211.: Tech. Assn. of Pulp & Paper Ind," vol. 37, No. 10, October 1954, 5 pages.

Pulp and Paper, Casey, vol. I, lnterscience Publishers, Inc., New York, 1952, page 429.

Paperrnaking Through Eighteen Centuries, Hunter, William Edwin Rudge, New York, 1930, page 316.

Owens-Oorningz Fiberglas Products for Papermaking, GLZAI, February 1954, page 10. 

4. A WATERLAID MICROPOROUS GLASS FIBER MAT SUITABLE FOR USE AS A BATTERY SEPARATOR CONSISTING ESSENTIALLY OF A CONTINUOUS LAYER HAVING A THICKNESS OF FROM .035 TO .050 INCH OF SHORT GLASS FIBERS, INCLUDING A HIGH PROPORTION OF FIBERS OF LENGTH RANGING FROM 0.008 TO 0.125 INCH, ARRANGED IN OVERRLAPPING, HAPHAZARD FELTED RELATION TO EACH OTHER WITH ONE EXPOSED SURFACE OF THE LAYER HAVING A PLURALITY OF SMALL, SPACED, PAEAKED PROJECTIOONS MADE UP OF THE FIBERS EXTENDING FROM THE LAYER AT AN ANGLE TO THE PLANE OF THE LAYER, THE PROJECTIONS BEING ARRANGED TO PROVIDE A PLURALITY OF CONTINUOUS CHANNELS EXTENDING ACROSS THE SURFACE OF THE MAT BETWEEN THE PROJECTIONS, THE PROJECTIONS EXTENSDING FROM THE LAYER A DISTANCE FROM ABOUT ONE HALF TO THE SAME AS THE THICKNESS OF THE LAYER AND HAVING THEIR PEAKS SPACED APART A DISTANCE GREATER THAN THE PROJECTION HEIGHT, THE FIBERS IN THE LAYER AND PROJECTIONS BEING BONDED TOGETHER BY A BINDER. 